feat(translation): on-device message translation

Add per-message translation that runs entirely on-device via the
Chromium built-in Translator + LanguageDetector APIs. Message text
never leaves the machine and never touches a cloud service, preserving
the E2EE guarantee. When the on-device engine is unavailable
(non-Chromium / mobile) the feature simply hides itself; there is no
network fallback.

- Engine abstraction (utils/translation): TranslationEngine interface
  plus a chromeTranslationEngine implementation (feature-detected,
  caches translator/detector instances, download-progress monitor).
  Pure lang-code helpers (normalize/sameLanguage/curated targets) with
  unit tests.
- Settings: translateTargetLang (default English) + autoTranslate
  (opt-in), with a Messages settings tile — a target-language select
  and an auto-translate switch, disabled with a note where unsupported.
- useMessageTranslation hook + shared per-event toggle atom-family and a
  persisted LRU cache so scrollback never re-translates.
- UI: a Translate / Show Original message-menu action, an inline
  "Translated from <lang> - Show original" chip, and a body swap in
  m.text/m.emote/m.notice that renders the translated text through the
  plain-text path (linkify + emoji) inside a dir=auto span for RTL.
- Auto-translate flips foreign messages whose model is already
  downloaded; first-time downloads keep the manual chip (user gesture).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/state/translation.test.ts

@@ -0,0 +1,61 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+
+// The module evaluates atomWithStorage(..., { getOnInit: true }) which reads
+// localStorage at load time. node has none — install a no-op mock, then import
+// dynamically (a static import would hoist above the mock).
+(globalThis as { localStorage?: unknown }).localStorage = {
+  getItem: () => null,
+  setItem: () => undefined,
+  removeItem: () => undefined,
+};
+
+const { addTranslation, findTranslation, makeCacheKey } = await import('./translation');
+
+const entry = (key: string, translated = `t-${key}`, fromLang = 'de') => ({
+  key,
+  translated,
+  fromLang,
+});
+
+test('makeCacheKey: eventId + normalized target', () => {
+  assert.equal(makeCacheKey('$abc', 'en-US'), '$abc:en');
+  assert.equal(makeCacheKey('$abc', 'EN'), '$abc:en');
+});
+
+test('addTranslation: prepends, newest first', () => {
+  const out = addTranslation([entry('a'), entry('b')], entry('c'));
+  assert.deepEqual(
+    out.map((e) => e.key),
+    ['c', 'a', 'b'],
+  );
+});
+
+test('addTranslation: de-dupes by key, moving to front (and updates value)', () => {
+  const out = addTranslation([entry('a', 'old'), entry('b')], entry('a', 'new'));
+  assert.deepEqual(
+    out.map((e) => e.key),
+    ['a', 'b'],
+  );
+  assert.equal(out[0].translated, 'new');
+});
+
+test('addTranslation: caps at max (newest kept)', () => {
+  const out = addTranslation([entry('a'), entry('b'), entry('c')], entry('d'), 3);
+  assert.deepEqual(
+    out.map((e) => e.key),
+    ['d', 'a', 'b'],
+  );
+});
+
+test('addTranslation: ignores empty key or translated', () => {
+  const start = [entry('a')];
+  assert.equal(addTranslation(start, entry('', 'x')), start);
+  assert.equal(addTranslation(start, entry('b', '')), start);
+});
+
+test('findTranslation: returns match or undefined', () => {
+  const list = [entry('a'), entry('b')];
+  assert.equal(findTranslation(list, 'b')?.key, 'b');
+  assert.equal(findTranslation(list, 'z'), undefined);
+});